The present invention relates generally to gas turbine engines, and, more specifically, to combustor liners therein.
A gas turbine engine includes a compressor for compressing air which is suitably mixed with a fuel and channeled to a combustor wherein the mixture is ignited for generating hot combustion gases which are channeled to a turbine joined by a shaft to the compressor. The turbine rotates and extracts energy from the combustion gases for powering the compressor, as well as producing useful work for propelling an aircraft in flight for powering a load, such as an electrical generator.
The liner must be suitably protected from the hot combustion gases for achieving a useful life during service. The art of combustor liner cooling is crowded with various types of arrangements for channeling a portion of compressed air from the compressor as the cooling fluid. However, any compressed air diverted from the compressor which is not mixed with fuel to form the combustion gases decreases the overall efficiency of the engine. Combustor liner cooling techniques therefore attempt to maximize cooling of the liner with minimum cooling air.
One type of liner cooling uses a plurality of spaced apart film cooling holes which extend through the liner for channeling the cooling air along the inner surface of the liner as a film barrier against the heat generated by the combustion gases. Another type of liner cooling includes impingement holes in a shroud spaced radially above the liner which direct cooling air in discrete jets against the outer surface of the liner for providing enhanced cooling thereof.
Gas turbine engines may be made more efficient by increasing the turbine inlet temperature of the combustion gases, which, of course, further increases the difficulty of suitably cooling the combustor liner. Conventional thermal barrier coatings (TBC) have been developed and are applied to the inner surface of the combustor liner for providing thermal insulation against the hot combustion gases. The TBC therefore reduces the cooling air requirements, which may be used to decrease the amount of cooling air used for a given combustion gas temperature, or allows an increase in the combustion gas temperature for increasing efficiency of the engine.
Combustor liner cooling is also a significant concern in the development of low NOx combustors since film cooling air for example quenches the temperature of the combustion gases along the inside surface of the liner which creates undesirable exhaust emissions. Conventional low NOx combustors built for industrial power generation gas turbine engines include water or steam injection for ensuring suitably low level of exhaust emissions, in particular NOx, for meeting federally mandated (EPA) exhaust emissions limits. It is desirable to eliminate water or steam injection and the attendant complexity and cost associated therewith which, in turn, increases the difficulty of suitably cooling a combustor to be used for such dry, low NOx operation.